is there a method to draw/fill a rounded rectangle?

Here is an implementation of your moon phase image (based on Bresenham's circle algorithm - yes, I'm able to learn!):

  let ScreenWidth  = g.getWidth(),  CenterX = ScreenWidth/2;
  let ScreenHeight = g.getHeight(), CenterY = ScreenHeight/2;

  g.setBgColor('#000000');
  g.clear(false);

  function drawMoonPhase (CenterX,CenterY, Radius, leftFactor,rightFactor) {
    let x = Radius, y = 0, Error = Radius;

    g.drawLine(CenterX-leftFactor*x,CenterY, CenterX+rightFactor*x,CenterY);

    let dx,dy;
    while (y <= x) {
      dy = 1 + 2*y; y++; Error -= dy;
      if (Error < 0) {
        dx = 1 - 2*x; x--; Error -= dx;
      }

      g.drawLine(CenterX-leftFactor*x,CenterY-y, CenterX+rightFactor*x,CenterY-y);
      g.drawLine(CenterX-leftFactor*x,CenterY+y, CenterX+rightFactor*x,CenterY+y);
      g.drawLine(CenterX-leftFactor*y,CenterY-x, CenterX+rightFactor*y,CenterY-x);
      g.drawLine(CenterX-leftFactor*y,CenterY+x, CenterX+rightFactor*y,CenterY+x);
    }
  }

  g.setColor('#FFFFFF');
  drawMoonPhase(CenterX,CenterY, 50, 1,-0.5);

leftFactor and rightFactor specify the actual phase that is to be shown:

• both variables range from -1...+1
  
• set both to 1 for a full moon
  
• set rightFactor = 1 and start with leftFactor = -1 for a waxing moon. Increase leftFactor until 1 for a full moon
  
• set leftFactor = 1 and start with rightFactor = 1 for a waning moon. Decrease rightFactor until -1
  
  

(see GitHub for the current source code and an animated demo)

Have fun!